A vertically oriented cylindrical tank that contains a liquid will have some outward pressure exerted on its walls at any point below the liquid surface. The pressure is directly related to the density of the liquid and the distance below the liquid surface.
The internal stress in the wall of a tank with a continuous side wall will be mainly horizontal tension which prevent the tank from expanding, and vertical compression which supports the weight of the tank wall. One can imagine the tank to consist of a number of hoops stacked one on top of another. Since each "hoop" is continuous, the tensile stress, which acts horizontally, is all that is needed to keep each "hoop" from expanding.
If a large hole is formed in the side of a vertical tank and then covered to prevent leakage, the tank wall to the right and left of the hole may bulge under internal pressure. Since the "hoops" which the hole passes through are not horizontally continuous, vertical components of tensile and compressive stress must exist to prevent the "hoops" from opening up around the aperture. The vertical tensile component will be largest in the segments of the tank wall adjacent to the hole on the right and left side, since the tensile stress tends to be directed tangentially around the hole in the tank. If the tank wall is so weak and the pressure is so great that the wall cannot withstand the vertical tension and the bending moment exerted on it, the tank wall will bulge.
Cylindrical, vertical tanks used for applications that require frequent manual cleaning of the inner surface, such as those used in manufacturing wine, must be equipped with a hermetically sealable access assembly, which is often referred to as a manway, on the side of the tank. This interruption in the tank wall will result in a large component of vertical tensile stress around the opening. It is desirable to manufacture the walls of a plastic tank relatively thin to minimize the amount of construction material and production time that is needed. On the other hand, thin walls are not stiff enough to prevent internal pressure from deforming the perimeter of the manway opening where the manway closure seals against the tank wall. As a result, the manway will not seal reliably and the tank will be unattractive in the bulged condition.
Thus, there is a need for a plastic tank with a thin wall, a manway, and a means of preventing the area around the manway from deforming due to internal pressure.
It is the object of this invention to fulfill this need without using protrusions or penetrating fasteners on the tank that bear a consequential load due to internal pressure.